Ask any soldier about the most essential part of their MRE, and I guarantee most of them will say either the hot sauce or the instant coffee. And during World War I, the reply would likely be the same minus the hot sauce.
Benjamin Franklin nailed it when he said, "Fatigue is the best pillow." True story, Benny. There's nothing like pushing your body so far past exhaustion that you'd willingly, even longingly, take a nap on a concrete slab.